1. Field of Invention
This invention relates to apparatus and methods for applying a blast of compressed air to an air filter of a granular plastic resin vacuum loader or receiver into which resin conveyance is powered by vacuum.
The invention further relates to apparatus and methods to dislodge resin pellets that may have stuck at the bottom of the loader or receiver, thereby preventing tight closure of a discharge flap and interfering with maintenance of the vacuum used to draw the granular plastic resin pellets into the loader or receiver.
The invention still further relates to loaders, for supplying granular plastic resin pellets to gravimetric blenders and other devices, which operate in a vertical orientation. Operation in the vertical orientation assures minimal and symmetrical stresses on the vacuum motor due to the vertical orientation of the vacuum motor shaft.
2. Description of the Prior Art
In plastics molding and extrusion facilities, it is typical for the granular plastic resin, which is to be molded or extruded in the facility, to be initially shipped to the facility in the form of pellets in large, heavy containers. These containers are delivered to the facility and stored there until the granular plastic resin pellets are required for use in the molding or extrusion process.
When the pellets of granular plastic resin material are needed, pellets of granular plastic resin material are removed from the storage container from time to time and conveyed, typically by a vacuum system, to where the pellets of granular plastic resin material are needed for molding or extrusion.
Plastic molding and extrusion facilities typically have vacuum lines running throughout the facility, in which vacuum of relatively low level, namely close to but below atmospheric pressure, is maintained. This vacuum is used to convey pellets of granular plastic resin material about the facility, thereby to deliver the pellets of granular plastic resin material to the loaders or receivers, which in turn function as temporary storage receptacles. The loaders or receivers typically provide the pellets of granular plastic resin material to gravimetric blenders or other devices. The gravimetric blenders blend the pellets of granular plastic resin material with additives, recycled resin pellets, and other ingredients to create a granular plastic resin material mix that is suitable for molding or extrusion.
A “receiver” for the pellets of granular plastic resin material includes a tube coupled to a vacuum source. Using vacuum, the tube conveys the pellets of granular plastic resin material into the receiver, where those pellets reside until the gravimetric blender or other device below the receiver needs additional pellets to prepare the blend of resin material for molding or extrusion. When a signal comes from the gravimetric blender, which signal may be provided by a level sensor within a storage bin of a gravimetric blender, a microprocessor associated with the receiver opens the bottom of the receiver. The pellets of granular resin plastic material then flow downwardly out of the receiver into the hopper of the gravimetric blender or other device. The pellets of granular plastic resin material are conveyed into the receiver by vacuum maintained within one of the tubes connected to the receiver. In this arrangement, the vacuum is drawn by a vacuum pump located elsewhere in the plastic molding or extrusion facility.
The receiver is typically equipped with a dust filter located between the space in the receiver in which the resin pellets reside and the suction intake for the vacuum line that pulls the vacuum from the receiver. The filter keeps dust and other particulate matter, traveling with the pellets of granular plastic material, from entering the suction intake through which the vacuum is drawn. Otherwise the dust would be drawn through the vacuum system, eventually reaching and entering the vacuum motor or pump located downstream, and likely fouling the vacuum motor or pump, thereby disrupting operation of the vacuum system and necessitating shutdown of the molding or extrusion operation.
A more common approach is to use a “loader”, which has a motor for drawing vacuum as a part of the loader. Prior art vacuum loaders are disclosed in U.S. Pat. Nos. 6,089,794 and 7,066,689, both issued in the name of Stephen B. Maguire, the inventor of record in the instant patent application. The loader is connected to a tube through which pellets of granular plastic material are conveyed typically by vacuum, but sometimes by air, throughout the plastic molding or extrusion facility. When the loader is actuated, the vacuum motor draws pellets of granular plastic resin material into a housing portion of the loader. The loader is equipped with a filter located between the inlet opening, through which the pellets of granular plastic resin material enter the loader, and a suction intake or opening leading to the vacuum motor. As with a receiver as described above, the filter keeps dust and other particulate matter, traveling with the pellets of granular plastic resin material, from entering the suction intake of the vacuum motor, fouling the motor and thereby interrupting operation.
Such dust filters are commonly used in both loaders and receivers and are positioned between the intake aperture, by which the pellets of granular plastic resin material enter the loader or receiver, and the suction intake for the vacuum motor (in the case of a loader) or the suction draw orifice (in the case of a receiver). During operation, air is typically drawn upwardly under vacuum in a vacuum loader to the suction intake for the vacuum motor. In a receiver, during operation, air is typically drawn under vacuum to the suction outlet of the receiver. In either case, the air drawn under vacuum passes through the filter, which removes dust particles and other contaminants from the air. As operation continues, dust and contaminants tend to clog the filter, reducing the amount of air that may be drawn by vacuum through the filter, thereby compromising operation of the vacuum motor in the case of a loader, or compromising the operation of the plant vacuum system in the case of a receiver.
To reduce this problem, it is known to force air through the filter, in a direction opposite that in which the vacuum is drawn when the loader or receiver is in operation. The air to be forced through the filter is provided by an air accumulator in conjunction with a typically solenoid-operated diaphragm valve. Typically, a reservoir is provided for accumulating some volume of air, with the reservoir being proximate the air filter. In a typical prior art loader or receiver, a diaphragm valve is typically positioned between the reservoir and the filter. When the valve is in a closed position, air, furnished from an air system that typically runs throughout the molding or extrusion facility and supplies compressed air at various points in the facility, for various purposes in the course of the molding or extrusion operation, fills the reservoir.
When the valve opens, air within the reservoir, being exposed to the vacuum environment within the loader or receiver, flows out of the reservoir, filling the volume between the filter and the vacuum intake, with some of the air passing through the filter. The reservoir supplies more air to the volume bounded in part by the filter and at a somewhat higher pressure than air drawn through the filter during normal operation of the loader or the receiver. This reverse flow of air, relative to the normal direction of air flow through the filter as drawn by the vacuum source, serves to clear the filter to some degree by removing some of the dust and entrained contaminants from the filter.
The solenoid operated diaphragm valve and air reservoir arrangements of the prior art provide improved operation over loaders and receivers that do not have such arrangements. However, the diaphragm valve/reservoir approach has limitations. Typically, the air from the reservoir flows rather gently and acts on only a relatively small area of the filter, because flow of air into the portion of the loader or reservoir between the filter and the vacuum source is necessarily limited by the size and cross-sectional area of the valve, through which the air must pass.
While providing a larger valve is one possible solution to the problem, significant cost increases and material handling problems are associated with larger valves, such that this is not a viable approach. Additionally the diaphragm valve approach often requires pilot valves, introducing more moving parts into the system, with a necessarily greater risk of failure. Moreover the risk of rupture of the diaphragm is always present.
Another approach to the problem is to provide multiple outlets using multiple solenoid actuated diaphragm valves for the high pressure air directed towards the filter. This again provides a costly and largely ineffective solution to this on-going problem plaguing the plastic fabrication industry.
Both loaders and receivers are used by the plastic industry to convey plastic resin pellets to process machines. On such loaders and receives, it is common to have a hinged flap at the bottom to allow discharge of the resin material, as well as to seal the bottom during loading to achieve the required vacuum to transport the resin material into the loader or receiver. In such loaders or receivers, a dump flap is sometimes also used as a level sensor to signal when the hopper requires an additional load of material. The dump flap is weighted in such a way so that it wants to close. Material that piles high in the hopper below the loader will block the dump flap from closing. When the pile drops, the dump flap swings closed and operates a device to signal the loader to load additional resinous material.
A problem is that static electricity will cause resin pellets to cling to the dump flap. When the dump flap closes, it is common for pellets to block the flap from closing completely. The result is an air leak, poor vacuum being drawn and little or no loading occurring during that load cycle due to the lack of sufficient vacuum to draw material into the loader or receiver.
One solution that has been used heretofore is addition of a soft rubber gasket to the dump flap, allowing pellets to sink into the rubber and still achieve a good seal at the dump flap. This approach does not always work and the gaskets degrade over time. These pellet adhesion problems can account for loss of up to fifty percent (50%) of the operating time of the loader or receiver.
Tilted loaders are known and used because their tilted orientation facilitates cleaning, but they are less than satisfactory in that they do not empty consistently, especially if the material being loaded is not an easily flowing material.